This invention was made in the course of, or under, a contract with the Energy Research and Development Administration. It relates in general to the melting of metals for casting and alloying operations and more particularly to the prevention of contamination of molten metals by crucible materials.
In a variety of metallurgical processes, metals are heated to temperatures at which they are reactive with crucible materials such as graphite, resulting in contamination of the finished product. Typically, such crucibles are provided with ceramic coatings which are substantially inert to the molten metals. Typical of such coatings is BN to protect molten aluminum from carbon contamination from graphite crucibles, as described in U.S. Pat. No. 3,245,674, to Baer, et al.
A particularly troublesome contamination problem is carbon contamination in uranium and uranium alloys such as uranium-16.6 atom percent niobium-5.6 atom percent zirconium. When uranium is melted in graphite crucibles with reactive metals such as niobium, titanium, vanadium, tantalum, molybdenum, zirconium, etc., the metals react with carbon and carbon oxides forming carbides which float to the surface of the melt causing the alloy to be deficient in these components, deleteriously affecting the mechanical properties and corrosion resistance of the alloy. While a known carbon content results in a predictable alloy composition, carbon pick-up from graphite crucibles used in induction melting produces unpredictable compositional variations. Accordingly, variation in alloy compositions in uranium alloys is directly correlatable to carbon contamination from graphite crucibles.